Cleaning Transfer Lines

How often should I clean lines that are used to transfer sap? I have two lines that I am dumping sap into that flows downhill and empties into a container. Sap is in it for just a short time before it empties and stays empty. Do I have to worry about bacteria growth in that empty line? Also what about those of you who are pumping sap from collection tanks into other tanks using a non permanent line. How often do you clean those lines and pump? Do I need to worry about bacteria growth there?

This will be my third season, I use one inch main line to transfer sap down a steep hill eight hundred feet long. A the end of the season I flush the line with fresh water about ten gallons seems to work. then I roll the line up for storage. 200 taps on pails.

Thought I would send this around one more time. Anyone out there have any advise on how often they are cleaning there non permanent transfer lines and pumps.